JPH10295081A - Voltage-dividing circuit with serial capacitor body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable use of an electrolytic capacitor having a low withstand voltage by adding a simple electrical circuit, setting a high resistance value for voltage division, in order to take a voltage balance of the serial bodies of electrolytic capacitors, and suppressing heating from a voltage-dividing resistor, thereby reducing the size of the circuit. SOLUTION: A collector of an NPN transistor 4a is connected to a positive pole of a capacitor 1a through a resistor 3e, an emitter of the NPN transistor 4a is connected to a negative pole of a capacitor 1a, and a resistor 3g is connected between the positive pole of the capacitor 1a and a base of the NPN transistor 4a. A collector of a PNP transistor 4b is connected to the negative pole of a capacitor 1b through a resistor 3f, an emitter of the PNP transistor is connected to a positive pole of the capacitor 1b, and a resistor 3h is connected between the negative pole of the capacitor 1b and the base of the PNP transistor 4b. Moreover, the emitters of the NPN transistor 4a and the PNP transistor 4b are connected to each other and as their bases are also connected each other as well.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a voltage dividing circuit of a series-connected capacitor series used in a power conversion circuit such as an inverter for driving a motor or an uninterruptible power supply.

[0002]

2. Description of the Related Art In a power converter such as an inverter for driving a motor or an uninterruptible power supply, an AC input is rectified by a converter circuit, converted into DC, and then converted again into a desired AC by an inverter circuit and output. Is performed. At this time, after rectification by the converter circuit, a capacitor is connected to smooth the direct current with a small pulsation voltage.
DC smoothing generally requires a capacitor having a large capacitance, and generally, a small electrolytic capacitor having a large capacitance is used. The DC voltage after rectification by the converter circuit depends on the number of phases and voltage of the AC input, but is approximately 1.3 times the AC input voltage. In a high-power power conversion circuit, a three-phase 400 V may be used as an AC input voltage, and in this case, the DC voltage is 500 V or more.

On the other hand, the withstand voltage of an electrolytic capacitor having a rated voltage of 450 V and a surge overvoltage of about 500 V is practically practical, and when the DC voltage is 500 V or more as described above, an appropriate withstand voltage is required. It is common to use electrolytic capacitors in series. When a voltage is applied to the electrolytic capacitor, a leakage current flows mainly due to an internal dielectric. Briefly, as shown in FIG. 4, the electrolytic capacitor 1 can be considered as an equivalent circuit having a capacitor component 2 and a parallel resistance component 3. (In fact, it has a small resistance component in series with the capacitor, but it can be ignored in the operation explanation of this case, so only the parallel resistance component is shown.)

Incidentally, the leakage current of an electrolytic capacitor having a high withstand voltage and a high capacitance used in a large-capacity power converter has a maximum current of 5 mA at normal temperature (about 20 ° C.) when a rated voltage is applied.
It is said that there are several times the individual differences. It also increases proportionally with temperature rise. That is, there is an individual difference in the parallel resistance component 3 shown in FIG. When electrolytic capacitors having such characteristics are used in series in a power conversion circuit, the shared voltage of the individual electrolytic capacitors becomes unbalanced due to the deviation of the parallel resistance component. Therefore, a voltage-dividing resistor is connected in parallel to each series-connected electrolytic capacitor,
The deviation of the internal parallel resistance component due to the leakage current is corrected.

FIG. 5 shows a case in which electrolytic capacitors 1a and 1b are connected in series.
Reference numerals a and 3b denote internal parallel resistance components, and reference numerals 3j and 3k denote voltage-dividing resistors connected in parallel outside the capacitor in order to correct the deviation of the parallel resistance components 3a and 3b. The added voltage-dividing resistors 3j and 3k are generally set to have the same resistance value because it is unclear what the deviation of the parallel resistance components 3a and 3b of the electrolytic capacitors connected in series is. In FIG. 5, among the parallel resistance components 3a and 3b, 3a is assumed to be large, and its resistance value is Rx, while 3b is Rm, and the resistance values of the voltage dividing resistors 3j and 3k are Rp. Rx and Rp and R
Assuming that the parallel combined resistance value of m and Rp is Rc1 and Rc2, Rc1 = Rx × Rp / (Rx + Rp) and Rc2 = Rm ×
Rp / (Rm + Rp), and the shared voltage of the individual electrolytic capacitors 1a and 1b is determined by the ratio of Rc1 and Rc2.

Here, the deviation of the parallel resistance component is as follows:
m, and the shared voltage of the electrolytic capacitor 1a having a large parallel resistance component is β
Rx = α × Rm, Rc1 =
It is represented by β × Rc2, and by substituting these into Rc1 and Rc2, the necessary resistance values Rp of the voltage dividing resistors 3j and 3k are as follows. Rp = α (β-1) × Rm / (α-β) (1)

For example, an electrolytic capacitor having a withstand voltage rating of 400 V and a surge voltage of 450 V is connected in series to form a DC 60
Let's calculate for the case of applying to a 0V circuit. Assuming that the leakage current of the electrolytic capacitor is 10 mA at the time of application of the rated voltage in consideration of temperature rise and the like, the parallel resistance component is 40 kΩ at 400 V / 10 mA, which is defined as Rm. On the other hand, assuming that the deviation of the leakage current is about three times, α = 3, and Rx is set to 120 k from 40 kΩ × 3.
Ω. Further, one of the shared voltages is assumed to be 200 V when one is allowed up to the rated withstand voltage of 400 V, and β = 400/200 = 2. In this case, from the above equation, Rp is 3 × Rm, that is, 120 kΩ. At this time, heat generated by the voltage dividing resistor 3j having a large applied voltage is 1.33.
W, the heat generated by one of the voltage dividing resistors 3k is 0.33W, and the total loss due to the heat is 1.7W.

[0008]

In recent years, miniaturization of devices has been regarded as very important, and power conversion circuits using electrolytic capacitors are no exception. As part of that,
Inverter circuits and the like are inevitably downsized, and downsizing individual components including electrolytic capacitors is becoming important. In general, if the external dimensions of an electrolytic capacitor are the same, the lower the withstand voltage, the greater the capacitance can be.The larger the ripple current capacity that determines the service life, the greater the withstand voltage can be. It is important to lower Therefore, even when the electrolytic capacitors are connected in series in an inverter circuit or the like as described above, the capacitors having the lowest withstand voltage of each electrolytic capacitor are selected and used.

However, if there is no margin in the withstand voltage of the electrolytic capacitor with respect to the applied voltage, the imbalance of the shared voltage due to the individual difference of the equivalent parallel resistance component inside the electrolytic capacitor becomes a serious problem. In order to suppress the shared voltage imbalance, it is necessary to reduce the resistance value of the voltage dividing resistor connected in parallel to the outside of the electrolytic capacitor and correct the influence of the deviation of the parallel resistance component of the electrolytic capacitor. Assuming that an electrolytic capacitor having a low withstand voltage is used, the calculation described in the related art will be performed and a comparison will be made. Assuming a case where the electrolytic capacitor having a rated voltage of 350 V and a surge voltage of 400 V is connected in series and applied to a DC 600 V circuit, the leakage current of the electrolytic capacitor is 10 mA and the parallel resistance component is 350 V / 10 mA = 35 kΩ as described above. This is defined as Rm. Similarly, it is assumed that the deviation of the leakage current is about three times, and α =
Set to 3.

Further, it is assumed that one of the shared voltages is 250 V when one is allowed up to the rated withstand voltage of 350 V, and β = 350/250 = 1.4. From equation (1), Rp is 0.75 × Rm, that is, 26.3 kΩ. At this time, the heat generated by the voltage dividing resistor 3j of the electrolytic capacitor having a high applied voltage is 4.6W, and the heat generated by the voltage dividing resistor 3k is 2.4W, which is 7W in total. By reducing the voltage by 50 V, it is necessary to reduce the voltage dividing resistance value, and the loss increases about four times. As described above, when the electrolytic capacitor is used with a reduced withstand voltage, it is necessary to reduce the voltage dividing resistance value, and accordingly, the heat generated in the voltage dividing resistance circuit increases, so that a large resistor having a large allowable loss is used. The use of such a capacitor results in the addition of a large resistor, which is contrary to the downsizing of the electrolytic capacitor, and an increase in loss due to resistance heating. In addition, when a large number of electrolytic capacitors are used in a large-capacity power converter, this problem becomes remarkable.

Although an electrolytic capacitor can secure a large capacitance, the tolerance of the capacitance is typically ± 20%. Therefore, when there is little margin in the withstand voltage, at the time of voltage fluctuation caused by power supply voltage fluctuation or load fluctuation,
The imbalance of the shared voltage caused by the tolerance of the capacitance of the electrolytic capacitor is also a problem. Regarding this, charge the electrolytic capacitor from the state where no voltage is applied,
This will be described with an example at the time of so-called initial charging.

In a power converter, when an alternating current is converted to a direct current by a converter circuit to initially charge an electrolytic capacitor,
Since the impedance of the electrolytic capacitor is very small at the beginning of charging, the charging current is suppressed through a low resistance for initial charging. As mentioned above, the charging resistance is low,
On the other hand, the resistance value of the voltage dividing resistor is set as large as possible to reduce heat loss. Therefore, the time constant determined by the initial charging resistance value and the capacitance of the electrolytic capacitor is larger than that determined by the voltage dividing resistor and the electrolytic capacitor. Generally, the capacitance of an electrolytic capacitor is several thousand μF, and the initial charging resistance is several Ω to several tens of Ω, whereas the voltage dividing resistance is several kΩ to
The time constant determined by the charging resistor and the capacitor is about several tens of ms, while the time constant determined by the voltage dividing resistor and the capacitor is about several tens of seconds, which is a very large difference.

FIG. 6 shows an equivalent circuit when the capacitor is initially charged, and FIG. 7 shows an operation waveform diagram. FIG. 6 is obtained by adding a DC power supply 5, a charging resistor 3n, and a closing switch 6 to the circuit of FIG. 5, and the same parts as those of FIG. 5 are denoted by the same reference numerals. The capacitor components of the electrolytic capacitors 1a and 1b have a smaller value of 2a and a larger value of 2b, and the internal parallel resistance components 3a and 3b and the voltage dividing resistors 3j and 3k are equal to each other. When the switch 6 is turned on and the voltage 2E of the DC voltage 5 is applied, the electrolytic capacitors 1a, 1b
7 rises as shown in (a) of FIG. 7. At this time, the shared voltage of 1a having a small capacitance is (b), and the other 1b is as shown in (c). As described above, since this charging time constant is short, the voltage balance at the beginning of the initial charging (at the time of Ts) is determined by the capacitance value of the electrolytic capacitor, and the shared voltage of the capacitor 1a having a small capacitance is increased. The voltage of 1b decreases. Subsequently, the shared voltage is gradually equalized by the time constant determined by the electrolytic capacitor and the voltage dividing resistor.

The shared voltage imbalance at the beginning of charging has a capacitance error of the electrolytic capacitor of ± 20%, ie, one of the two is 1.
It must be taken into account that the other has a ratio of 0.8 to 2. Therefore, in the example of FIG. 7, the peak voltage of (b) is 1.2E. At this time, if the time of exceeding the rated voltage is very short, it is possible to design within a range that does not exceed the surge withstand voltage of the electrolytic capacitor. However, as described above, it takes several tens of seconds to achieve a balanced state. Since it takes a long time, it is necessary to increase the rated voltage. As described above, since the capacitance varies about ± 20%, it is difficult to select an electrolytic capacitor having a low withstand voltage by the conventional method, and this is a factor that hinders downsizing of the circuit. Was.

[0015]

According to the present invention, a collector of an NPN transistor is connected to a positive electrode of a first capacitor via a first resistor. The emitter of the NPN transistor is connected to the negative electrode of the first capacitor, the second resistor is connected between the positive electrode of the first capacitor and the base of the NPN transistor, and the collector of the PNP transistor is connected to the third resistor. And the emitter of the PNP transistor is connected to the positive electrode of the second capacitor, and the negative electrode of the second capacitor is connected to the negative electrode of the second capacitor.
A fourth resistor is connected between the bases of the NP transistors, the emitter of the NPN transistor is connected to the emitter of the PNP transistor, and the base of the NPN transistor is connected to the base of the PNP transistor. As described in claim 2, a plurality of the above configurations can be provided.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an example in which the present invention is applied to a two-series circuit of electrolytic capacitors 1a and 1b. The collector of an NPN transistor 4a is connected to the positive electrode of a capacitor 1a via a resistor 3e, and the emitter of the NPN transistor 4a is Connected to the negative electrode of the capacitor 1a,
3g between the positive electrode of the transistor and the base of the NPN transistor 4a
Connect. The collector of the PNP transistor 4b is connected to the negative electrode of the capacitor 1b via the resistor 3f.
The emitter of the transistor 4b is connected to the positive electrode of the capacitor 1b, and the negative electrode of the capacitor 1b is connected to the PNP transistor 4b.
A resistor 3h is connected between the bases of b. Further, the emitter and the base of the NPN transistor 4a and the PNP transistor 4b are connected. The connection point between the emitters is (a), and the connection point between the bases is (b).

Here, the parallel resistance components 3a and 3b in the series body of the electrolytic capacitor shown in FIG. 1 have a large value 3a (the resistance value is Rx) and a small value 3b (the resistance value is R), as in FIG.
m), the potential at the point (a) viewed from the negative electrode of the electrolytic capacitor is smaller than １ ／ of the applied voltage, while the potential at the point (b) is と of the applied voltage by the resistors 3g and 3h. Has become. Point (a) is the emitter terminal of transistors 4a and 4b, and point (b) is the base terminal of transistors 4a and 4b. In this case, a base current is supplied to NPN transistor 4a to conduct. FIG. 2 shows an equivalent circuit when the operation of the transistor is an ideal switch operation. Since the resistors 3a and 3e form a parallel circuit due to the conduction of the transistor 4a, equalizing the shared voltage of the electrolytic capacitor means equalizing the combined resistance value of the resistors 3a and 3e and the resistance value of the resistor 3b. That is, the resistor 3e
Assuming that the internal resistance deviation of the electrolytic capacitor shown in the figure is α, that is, Rx = αRm, the required value of Rf is Rf = α × Rm, where Rx × Rf / (Rx + Rf) = Rm. / (Α-1), and Rf does not need to be equal to or less than Rm.

Here, the applied voltage 600
When calculating with V, Rm = 35 kΩ and α = 3, the required Rf is 52.5 kΩ and the heat generation is 1.7 W.
Further, since the transistor 4b is not conducting, the resistor 3f in FIG. 1 is separated from the circuit and does not generate heat. Since the potential at the point (b) is used as a reference for the divided voltage, the resistors 3g and 3h need a certain degree of accuracy. However, since only the base current is supplied, the resistors 3g and 3h can have a high resistance value.
Therefore, the heat loss due to the resistance can be reduced to approximately 1/4 as compared with the above-described example. Of course, when the relationship of the resistance deviation of the electrolytic capacitor is reversed, FIG.
The potential at the point becomes higher than the potential at the point (b),
A base current is supplied to the P transistor 4b to make it conductive,
Obviously, the operation is the same as described above.

Further, according to the voltage dividing circuit of this embodiment,
The resistance value for voltage division can be set small without increasing the loss of the resistance value Rf of the voltage division resistance of FIG. 1, and the imbalance correction operation can be performed quickly. In a state where the voltage dividing resistance is set small, for example, when a shared voltage imbalance occurs due to a capacitance deviation of the electrolytic capacitor at the time of initial charging or the like, the points (a) and (b) shown in FIG. The potential difference increases with time, and the current supplied to the base of the transistor 4a or 4b also increases. Therefore, the current flowing through the collector of the transistor, that is, the current flowing through the voltage dividing resistor 3e or 3f also becomes large because the voltage dividing resistor is set small, thereby rapidly correcting the state of unbalanced voltage of the electrolytic capacitor.

As the shared voltage imbalance of the electrolytic capacitor is eliminated, the potential difference between the points (a) and (b) disappears, the base current of the transistor decreases, and the collector current also decreases. Only the difference due to the parallel resistance component deviation flows through the voltage dividing resistor and the transistor. At this time, the applied voltage of the electrolytic capacitor is balanced, and the applied voltage of the voltage dividing circuit is applied to the voltage dividing resistor only by the voltage drop due to the above-mentioned difference current, and the remaining voltage is applied to the transistor. Therefore, the loss in the voltage dividing circuit including the transistor becomes equal to the voltage dividing resistance of the circuit shown in FIG. 2, and the loss does not increase.

Next, an embodiment according to claim 2 is shown in FIG. FIG. 3 shows a configuration in which the voltage dividing circuit shown in FIG. 1 is added to a plurality of series electrolytic capacitors in a pair with the series electrolytic capacitors. In the figure, 1a1 to 1a3, 1b1 to 1b
3 is an electrolytic capacitor, 2a1-2a3, 2b1-2b3
Is a capacitor component, 3a1-3a3, 3b1-3b3 are parallel resistance components, 3e1-3e3, 3f1-3f3, 3g
1-3g3, 3h1-3h3 are voltage dividing resistors, 4a1-4a
Reference numerals 3, 4b1 to 4b3 are transistors. Since the operation of each voltage dividing circuit is not different from that described in FIG. 1, its usefulness is clear.

[0022]

According to the present invention, by simply adding a simple electric circuit, it is possible to set a large resistance value for voltage division for balancing the voltage of the series body of electrolytic capacitors, and to suppress heat generation of the voltage division resistance. In addition, it is possible to use an electrolytic capacitor having a low withstand voltage while reducing the size. Also, the resistance value of the voltage dividing resistor can be reduced without increasing the circuit loss, so that the voltage dividing imbalance due to a capacitance error occurring at the time of initial charging or the like can be rapidly corrected, and an electrolytic capacitor can be used. It is also possible to carry out a design in consideration of using the surge withstand voltage region of FIG.

[Brief description of the drawings]

FIG. 1 is a circuit diagram for explaining claim 1 of the present invention.

FIG. 2 is an equivalent circuit diagram for explaining claim 1 of the present invention.

FIG. 3 is a circuit diagram for explaining claim 2 of the present invention.

FIG. 4 is a circuit diagram for explaining a conventional example.

FIG. 5 is a circuit diagram for explaining a conventional example.

FIG. 6 is a circuit diagram for explaining a problem of the conventional example.

FIG. 7 is a waveform chart for explaining a problem of the conventional example.

[Explanation of symbols]

··· electrolytic capacitor 2, 2a, 2b ··· capacitor component 3, 3a, 3b ··· parallel resistance component 3e, 3f, 3g, 3h, 3j, 3k, 3n ··· · Resistance 4a, 4b · · · Transistor 5 · · · DC power supply 6 · · · ON switch

Claims (2)

[Claims] 1. A collector of an NPN transistor is connected to a positive electrode of a first capacitor through a first resistor.
The emitter of the N transistor is connected to the negative electrode of the first capacitor, and the positive electrode of the first capacitor and the NPN
A second resistor is connected between the bases of the transistors, and a PNP
The collector of the transistor is connected through a third resistor to the negative electrode of the second capacitor, the emitter of the PNP transistor is connected to the positive electrode of the second capacitor, and the negative electrode of the second capacitor is connected to the negative electrode of the PNP transistor. Connecting a fourth resistor between the bases, connecting the emitter of the NPN transistor and the emitter of the PNP transistor,
A voltage dividing circuit of a capacitor series body, wherein a base of the NPN transistor is connected to a base of the PNP transistor. 2. A voltage dividing circuit for a capacitor series body comprising a plurality of series-connected voltage dividing circuits for the capacitor series body according to claim 1.